The present invention relates to an interlayer dielectric film that is low in relative dielectric constant and good in mechanical strength, thermal resistance, and adhesion to a substrate, a method for forming such an interlayer dielectric film, and a method for forming an interconnection on such an interlayer dielectric film.
As organic polymer films usable as the interlayer dielectric film of VLSI, known are polymer films formed by polymerizing aromatic molecules, which exhibit good thermal resistance, polymer films made of polyimide derivatives, polymer films made of polyallylether derivatives, polymer films made of polyquinoline derivatives, polymer films made of polyparaxylene derivatives, and the like.
The above organic polymer films, which contain carbon as a main component, are low in polarizability of component molecules and thus low in relative dielectric constant, compared with silicon oxide films that have been conventionally used as the interlayer dielectric film of VLSI. Due to this feature, polymer films have received attention as interlayer dielectric films having a low relative dielectric constant. The relative dielectric constant of an organic polymer film containing carbon as a main component is about 2.4 to about 3.0, which is lower than that of a silicon oxide film that is about 3.3 to about 4.5. As an exception, however, among the silicon oxide films there is known a kind of organic SOG film containing an organic component that has a relative dielectric constant of about 2.9.
In recent years, further reduction in the relative dielectric constant of the interlayer dielectric film has been desired, and for this purpose, porous interlayer dielectric films have been examined.
When an interlayer dielectric film is porous, the relative dielectric constant of the film is low. However, a porous interlayer dielectric film has a new problem that the mechanical strength, the thermal resistance, and the adhesion to a substrate of the interlayer dielectric film are lowered.
The above problem arises due to the structural defect of the conventional porous interlayer dielectric film that it is obtained by reducing the density of cross-linking of an organic polymer. This structural defect will be discussed as follows.
The mechanical strength of an organic polymer film is greater as the density of cross-linking of the film is higher. However, the conventional organic polymer film is made porous by cutting cross-linked sites of the organic polymer, resulting in partially cutting the network of molecules by which the mechanical strength of the organic polymer film is maintained. This inevitably lowers the mechanical strength. The interlayer dielectric film having a reduced mechanical strength causes a problem of destroying an interconnection structure when the interlayer dielectric film is flattened by chemical mechanical polishing (CMP).
The cutting of cross-linked sites of the organic polymer also causes the following problem. An interlayer dielectric film made of such an organic polymer film becomes softened when it is subjected to heat treatment at a later stage. This causes deformation or destruction of a multi-layer interconnection structure.
In addition, the cutting of cross-linked sites of the organic polymer also cuts cross-linked sites between the organic polymer film and a substrate. This results in reducing the adhesion of the interlayer dielectric film to the substrate.
Moreover, in the conventional porous organic polymer film, statistical variation in the size (size distribution) of pores in the film is unavoidable and the dispersion of the pores is not uniform. Accordingly, the resultant interlayer dielectric film is no more uniform in quality, and thus fails to respond to the demand for finer VLSI.
If VLSI is made finer while using the conventional porous organic polymer film, there will inevitably exist pores having a size close to a design rule. Existence of such pores will cause a problem of pattern defect.
Furthermore, a number of pores are formed continuously in the conventional porous organic polymer film. This unavoidably causes a trouble of water, etching gas, a cleaning solution, or the like entering the interlayer dielectric film, thereby deteriorating the quality of the film.